1. Field of the Invention
The present invention relates to helical scanning tape recording methods and apparatus therefor.
2. Description of the Prior Art
A helical scan approach to magnetic recording is now used universally in the videotape recording industry, and the same approach has also been used for high data rate digital recorders. In a typical helical scan recorder for video tape, the recording tape is taken from a supply reel, guided onto a rotating head wheel, wrapped partially around the head wheel, and then guided off the head wheel onto a take-up reel. Commonly, the supply and take-up reels are mounted and supported in a form known as a video tape cassette. The path of the tape as wrapped around the head wheel is skewed, thus following a helical path on the periphery of the head wheel (giving rise to the term of art "helical scan"). One or more magnetic heads protrude slightly from the head wheel and rotate with the head wheel to engage the magnetic tape along a diagonal path on the tape from edge to edge and along a length of the tape equal approximately to the amount of wrap of the tape on the head wheel. For each pass of the magnetic head across the tape (as the head wheel rotates relative to the tape), the tape advances slightly along the helical path so that each subsequent diagonal path on the tape traced by the magnetic head lies adjacent to the previous diagonal path. A significant advantage realized by helical recording techniques is the attainment of a head-to-tape speed of hundreds of inches per second, allowing very high frequencies to be recorded. Another advantage of a helical scan system is to allow long uninterrupted lines ("tracks") of information relative to the width of the tape while at the same time maximizing the area of the tape covered with recording tracks.
The use of helical scanning is especially useful where multiple tracks are recorded simultaneously in order to achieve a high data transfer rate. While this method lends itself to scanning formats in which as many as four heads are in contact with the magnetic tape simultaneously, the difficulty of making electrical contact with rotating heads (either by rings and brushes, or by rotating transformers) limit the number of heads which can be practically employed. This in turn limits the maximum data transfer rate obtainable with helical scanning of magnetic tape. In addition, with typical track widths of 0.001 inch width or less, the difficulties of aligning multiple heads on a rotating head wheel are formidable.
Recent developments in information storage technology have resulted in a variety of techniques of recording and reading information by optical or magneto-optic means. At present, optical recording technology is being utilized in the consumer market for distribution of pre-recorded television programming (optical video disk) and pre-recorded high fidelity sound (compact disk). These applications demonstrate the reliability of optical recording techniques. In addition, applications are emerging for the write-once optical disks by commercial and industrial users for storage of digital data, including the storage of digitized images. Commercial introduction of erasable media, such as magneto-optic disks, will further enhance the utilization of optical recording systems and will make possible the storage of large numbers of digitized images within relatively small disk drives.
The principal virtues of optical disk recording are high storage density (approximately 2.times.10.sup.8 bits per square inch), high speed random access to large data stores, and a non-contact (i.e., non-wearing) machine-media interface which is mechanically forgiving. These virtues translate to digitized image storage capabilities which combine high capacity, small size, and low cost.
While optical disk recording provides unprecedented storage density and access to data, the surface area of a disk is limited by the constraint of a convenient and manageable disk diameter. Combinations of multiple disks in a "juke box" format circumvents this constraint to a certain extent, but is an expensive solution to the problem.
Optical recording on a tape medium, on the other hand, provides a much greater surface area than on a manageable disk, since the area of a tape is limited only by the size of the reel on which it is wound. Of course, use of a tape for data storage provides serial access to the data and may be limited to applications in which access time is not important or where the data is always handled in continuous, real time data streams. Numerous applications exist for this kind of data handling, including long-term data storage and back-up for disks. Digitized visual images generally generate continuous data streams because digital values are generated from analog image scanning devices.
Attempts have been made to combine the advantages of helical scanning with optical tape. In such arrangements, the tape follows a helical path around a head wheel, with the rotating magnetic head of a helical magnetic scanner replaced by rotating some or all optical components of a laser-driven optical scanning system. The head wheel, containing certain optical components, has in it a transparent window or opening to allow the laser beam to reach the tape from within the head wheel. Examples of such schemes are seen in Bell et al. U.S. Pat. No. 4,661,941, Hudson U.S. Pat. No. 4,633,455 and Higashiyama U.S. Pat. No. 4,525,828. These patents show optical tape recording systems in which the beam from an external laser is projected into the interior of a head wheel along the axis of rotation of the head wheel with an assortment of mirrors, beam-splitters and the like, some of which rotate with the head wheel to produce the necessary optical scan. As illustrated by the complex optical arrangements shown in these patents, rotating optics are not easily implemented. In addition, since the read/write laser beam reaches the tape from inside the head wheel, the data borne on the tape must be accessed through or read off of that side of the tape which contacts the surface of the rotating head wheel, which has a surface velocity greatly in excess of the surface velocity of the tape. The surface of the tape in contact with the head wheel, which is usually the critical surface of the tape for information transfer purposes, is thus subject to possible damage and data loss through contact with the high speed head wheel.
Although helical scanning of magnetic tape is now a standard procedure (using rotating heads), the ability to quickly transfer data is limited because of physical constraints and alignment problems, as discussed above. The ability to obtain a higher data transfer rate for magnetic tape using helical scanning is desired, however, since magnetic tape is easy to handle, involves well-known recording technology, and is a relatively inexpensive medium. In addition, the data recording and retrieval heads for magnetic tapes are relatively inexpensive, compared to optics.